This invention relates to a control device for controlling operation of a stepping motor.
Various types of stepping motors or pulse motors have been widely utilized in connection with electronic appliances. For example, a stepping motor is mounted in an electronic sewing machine for incremental feed of a fabric to be sewn. The stepping motor is driven in response to stitch control data which are in advance stored in a memory for each stitch of a stitch pattern.
A control device has thus been used for controlling operation of a stepping motor which typically involves a circuit shown in a block diagram of FIG. 1. A current at A--A phase of the stepping motor is detected by a resistor R.sub.1 whereas a current at B--B phase is detected by another resistor R.sub.2, the results of which being input to first and second comparators 10 and 11 respectively for comparison with reference voltage Vref. The comparative results obtained at the first and second comparators 20, 21 are alternately and sequentially output to a stepping motor drive circuit 23 in synchronism with a clock signal generated from a trigger pulse oscillator 22. Thus, the stepping motor has to be driven by a constant current.
With the conventional control device, however, torque characteristics of the stepping motor would be varied by variation of voltages Va.sub.1, Va.sub.2 for driving the stepping motor as shown in FIG. 2(a). Such variation of the driving voltages Va.sub.1, Va.sub.2 would be caused by, for example, a voltage variation in a line supply (which may be varied in the order of .+-.10%) and a voltage variation in a transformer (in the order of .+-.5%). The torque characteristics of the stepping motor would also be varied by variation of winding currents I.sub.1, I.sub.2 of the stepping motor, as shown in FIG. 2(b), resulting from factors residing in the control device, for example, inherent errors in design of resistance values of potential dividing resistors (in the order of .+-.7%) for providing the reference voltage Vref, as well as the current detecting resistors R.sub.1, R.sub.2 (in the order of .+-.2%). Moreover, the load of the stepping motor would undergo a change by an error in accuracy of assembled components and variations of circumferential temperatures.
By virtue of these variation factors, a current to be fed to the stepping motor should have been determined to be considerably higher than a theoretical value so that the stepping motor could surely be driven even under the worst conditions. This would require a large mass of electric power to be supplied to the stepping motor and often overheat the stepping motor.